The present invention relates to a nematic liquid crystal composition. More specifically, the invention relates to a liquid crystal composition for an active matrix liquid crystal display device and to a liquid crystal display device using the liquid crystal composition.
To date, a transmission-type active matrix liquid crystal display device (AM-LCD) using a backlighting as a light source has been used practically as a device enabling full color display. As this backlighting has a weak point to consume considerable electric power, a digital still camera and a digital video camera equipped with such AM-LCD have an inconvenience that they cannot be used for a long time.
To solve the problem of the electric power consumed, a reflection-type AM-LCD has been developed. In the reflection-type AM-LCD, as reported by S.-T. Wu, C.-S. Wu, C.-L. Kuo et al. in SID 97 Digest/643, light passes twice through a liquid crystal layer so that the product (xcex94nxc2x7d) of thickness of the liquid crystal layer (d) and optical anisotropy (xcex94n) should be set smaller. For this reason, xcex94n required for the liquid crystals of the reflection TN-type AM-LCD is 0.07 and less compared to 0.075xcx9c0.120 of xcex94n required for the liquid crystals of a conventional transmission TN-type AM-LCD.
Besides xcex94n, characteristics of the liquid crystal composition required for the reflection-type AM-LCD are as follows, just like for the conventional transmission-type AM-LCD:
(1) to have high specific resistance and a large voltage holding ratio in order to keep high contrast of the LCD;
(2) to have a broad temperature range exhibiting a nematic phase in order to make outdoor use of the LCD possible, namely, to have a higher upper-limit of temperature exhibiting a nematic phase and a lowere lower-limit of temperature exhibiting a nematic phase;
(3) to have low threshold voltage in order to reduce electric power consumed in the LCD; and
(4) to have low viscosity in order to accelerate display speed of the LCD.
Literature disclosing liquid crystal compounds or liquid crystal compositions which is thought to be applicable to the AM-LCD can be illustrated as follows: WO 96/23851, Japanese Patent Kokai Hei 9-71779 (1997), Japanese Patent Kokai Hei 9-110981 (1997), DE 19629812 A1 and WO 98/17664. However, the liquid crystal compositions disclosed in these literatures, as shown in the comparative examples of the present application, have deficiencies such as large xcex94n, a high lower-limit of temperature exhibiting a nematic phase in spite of xcex94n being relatively small, or a small voltage holding ratio so that they are insufficient to use for the reflection-type AM-LCD.
Thus the liquid crystal compositions are always requested improvement though they have been keenly investigated depending on various purposes.
The object of the present invention is to provide a liquid crystal composition which has, in particular, a high upper-limit of temperature exhibiting a nematic phase, a low lower-limit of temperature exhibiting a nematic phase, and small optical anisotropy while satisfying general properties required for an AM-LCD.
The inventors of the present invention, after enthusiastic investigation of the liquid crystal composition using various liquid crystal compounds to solve these subjects, have found that the liquid crystal composition described in the present invention as shown later can be used for the reflection TN-type AM-LCD with possibility of displaying full color so that the object intended can be achieved. Followings are detailed explanation of the present invention.
The first of the present invention is a liquid crystal composition characterized in that it comprises at least one compound expressed by formula (1) as the first component, at least one compound expressed by formula (2) as the second component, and at least one compound expressed by either formula (3-1) or (3-2) as the third component. 
wherein R1, R3, R4 and R5 each independently represent alkyl having 1 to 10 carbon; R2 represents alkyl or alkoxy having 1 to 10 carbon, or xe2x80x94COOxe2x80x94R6; Z1 represents a single bond or xe2x80x94C2H4xe2x80x94; and R6 represents alkyl having 1 to 10 carbon.
The second of the present invention is a liquid crystal composition characterized in that it comprises as the first component at least one compound expressed by formula (1) in 5 to 95% by weight based on the total weight of the composition; it comprises as the second component at least one compound expressed by formula (2) in 5 to 25% by weight based on the total weight of the composition; and it comprises as the third component at least one compound expressed by either formula (3-1) or (3-2) in 5 to 70% by weight based on the total weight of the composition.
The third of the present invention is a liquid crystal composition characterized in that it comprises, as the fourth component in addition to the first, the second, and the third components, at least one compound selected from the group of compounds expressed by formulae (4-1) to (4-3) in 85% and less by weight based on the total weight of the composition. 
wherein R7, R8 and R9 each independently represent alkyl having 1 to 10 carbon; X1, X3, X4, and X5 each independently represent H or F; X2 represents Cl, F, or alkoxy having 1 to 10 carbon; Z2 represents xe2x80x94COOxe2x80x94, xe2x80x94C2H4xe2x80x94 or a single bond; Z3 and Z4 each independently represent xe2x80x94C2H4xe2x80x94 or a single bond; with the proviso that Z2 is always xe2x80x94COOxe2x80x94 when X1 and X3 are H simultaneously.
The fourth of the present invention is a liquid crystal composition characterized in that an upper-limit of temperature exhibiting a nematic phase is 70xc2x0 C. or more, a lower-limit of temperature exhibiting a nematic phase is xe2x88x9220xc2x0 C. and less, and optical anisotropy is 0.07 and less.
The fifth of the present invention relates to a liquid crystal display device using any liquid crystal composition described in the first to the fourth of the present invention.
Now, we explain the preferable embodiment of compounds constituting the liquid crystal composition of the present invention. The compounds expressed by following formulae (1-1) to (1-3) are preferably used among those expressed by formula (1). 
wherein R and Rxe2x80x2 each independently represent linear alkyl having 1 to 10 carbon.
The compounds expressed by following formula (2-1) are preferably used among those expressed by formula (2). 
wherein R represents linear alkyl having 1 to 10 carbon.
The compounds expressed by following formula (3-1-1) or (3-1-2) are preferably used among those expressed by formula (3-1). 
wherein R represents linear alkyl having 1 to 10 carbon.
The compounds expressed by the following formula (3-2-1) are preferably used among those expressed by formula (3-2). 
wherein R represents linear alkyl having 1 to 10 carbon.
The compounds expressed by the following formulae (4-1-1) to (4-1-7) are preferably used among those expressed by formula (4-1). 
wherein R and Rxe2x80x2 each independently represent linear alkyl having 1 to 10 carbon.
The compounds expressed by following formulae (4-2-1) to (4-2-5) are preferably used among those expressed by formula (4-2) 
wherein R represents linear alkyl having 1 to 10 carbon.
The compounds expressed by the following formula (4-3-1) are preferably used among those expressed by formula (4-3). 
wherein R represents linear alkyl having 1 to 10 carbon.
Next, we explain a roll of each compound constituting the liquid crystal composition of the present invention.
The compound expressed by formula (1) has features that dielectric anisotropy is nearly zero, a value of specific resistance is large, the upper-limit of temperature exhibiting a nematic phase is relatively low, viscosity is low, and optical anisotropy is considerably small. For this reason, the compound expressed by formula (1) is specifically used for the object to reduce viscosity and to minimize optical anisotropy while maintaining a high voltage holding rate of the liquid crystal composition.
The compound expressed by formula (2) has features that dielectric anisotropy is large, a value of specific resistance is large, the upper-limit of temperature exhibiting a nematic phase is high, and optical anisotropy is relatively small. For this reason, the compound expressed by formula (2) is specifically used for the object to raise the upper-limit of temperature exhibiting a nematic phase and to lower threshold voltage, while maintaining high voltage holding rate and small optical anisotropy of the liquid crystal composition.
The compounds expressed by formulae (3-1) and (3-2) have larger dielectric anisotropy and a larger value of specific resistance than the compound expressed by formula (2). Also they have features that the upper-limit of temperature exhibiting a nematic phase is not higher than the compound expressed by formula (2) and is higher than the compound expressed by formula (1), and optical anisotropy is relatively small. For this reason, the compounds expressed by formulae (3-1) and (3-2) are specifically used for the object to further lower the threshold voltage, while maintaining a large voltage holding ratio and small optical anisotropy.
The combination of the compounds expressed by formulae (1) , (2), and (3-1) or (3-2) makes possible to obtain the liquid crystal composition especially in which the upper-limit of temperature exhibiting a nematic phase is high, the lower-limit of temperature exhibiting a nematic phase is low, and optical anisotropy is small, while satisfying general features of the present invention which is required for the AM-LCD.
For example, the compositions composed only of the compounds expressed by formula (1), only of the compounds expressed by formula (2), and only of the compounds expressed by formula (3-1) or (3-2) are all not able to achieve the object of the present invention. The compositions composed only of the combination of the compounds expressed by formula (1) and the compounds expressed by formula (2), only of the combination of the compounds expressed by formula(l) and the compounds expressed by formula (3-1) or (3-2), and only of the combination of the compounds expressed by formula (2) and the compounds expressed by formula (3-1) or (3-2), can not be the composition having in particular the low lower-limit of temperature exhibiting a nematic phase which is the purpose of the present invention.
The compound expressed by formula (4-1) has zero or positive dielectric anisotropy, a large value of specific resistance, and relatively small optical anisotropy. The compound has a feature to suppress the exhibition of a smectic phase at low temperature by the addition of it to a liquid crystal composition. For this reason, the compound expressed by formula (4-1) can be used for the object particularly to adjust the lower-limit of temperature exhibiting a nematic phase or to adjust the threshold voltage, while maintaining a large voltage holding ratio and small optical anisotropy of the liquid crystal composition.
The compound expressed by formula (4-2) has features that dielectric anisotropy is relatively large, a value of specific resistance is high, the upper-limit of temperature exhibiting a nematic phase is relatively high, and optical anisotropy is relatively small. For this reason, the compound expressed by formula (4-2) of the present invention can be used for the object particularly to raise the upper-limit of temperature exhibiting a nematic phase and to further lower threshold voltage, while maintaining a large voltage holding ratio and small optical anisotropy.
The compound expressed by formula (4-3) has features that dielectric anisotropy is relatively large, a value of specific resistance is large, the upper-limit of temperature exhibiting a nematic phase is relatively high, and optical anisotropy is relatively small. For this reason, the compound expressed by formula (4-3) can be used for the object particularly to further raise the upper-limit of temperature exhibiting a nematic phase, and to control threshold voltage, while maintaining large voltage holding ratio and small optical anisotropy.
Next, we explain a preferable ratio of the components constituting the liquid crystal composition of the present invention and the reason thereof.
The compound expressed by formula (1) may sometimes raise the lower-limit of temperature exhibiting a nematic phase in the liquid crystal composition when a large amount of it exists in the composition. For this reason, the ratio of the compound expressed by formula (1) in the composition is preferably 95% and less by weight, more preferably 75% and less by weight, and further more preferably 65% and less by weight, respectively, based on the total weight of the liquid crystal composition. In order to minimize the optical anisotropy of the liquid crystal composition, the ratio of the compound expressed by formula (1) in the liquid crystal composition is preferably 5% or more by weight, more preferably 10% or more by weight, respectively, based on the total weight of the liquid crystal composition. In order to minimize optical anisotropy of the liquid crystal composition and to reduce viscosity, the ratio of the compound expressed by formula (1) is further more preferably 20% or more by weight based on the total weight of the liquid crystal composition.
The compound expressed by formula (2) may sometimes raise the lower-limit of temperature exhibiting a nematic phase when a large quantity of it exists in the composition. For this reason, the ratio of the compound expressed by formula (2) in the composition is preferably 25% and less by weight, and more preferably 20% and less by weight, respectively, based on the total weight of the liquid crystal composition. In order to relatively lower threshold voltage of liquid crystal composition and to raise the upper-limit of temperature exhibiting a nematic phase, the ratio of the compound expressed by formula (2) in the liquid crystal composition is preferably 5% or more by weight based on the total weight of the liquid crystal composition.
The compound expressed by formula (3-1) and the compound expressed by formula (3-2) may sometimes raise the lower-limit of temperature exhibiting a nematic phase when a large amount of it exists in the composition. For this reason, the ratio of the compound expressed by formula (3-1) or (3-2) in the liquid crystal composition is preferably 70% and less by weight, more preferably 60% and less by weight, and further more preferably 50% and less by weight, respectively based on the total weight of the liquid crystal composition. In order to further lower the threshold voltage of liquid crystal composition and to lower the lower-limit of temperature exhibiting a nematic phase, the ratio of the compound expressed by formula (3-1) or (3-2) in the liquid crystal composition is preferably 5% or more by weight based on the total weight of the liquid crystal composition.
In the use of a forth component in the liquid crystal composition of the present invention, the compound expressed by formulae (4-1) to (4-3) may raise the lower-limit of temperature exhibiting a nematic phase when a large amount of it exists in the liquid crystal composition. For this reason, the ratio of the compound expressed by formulae (4-1) to (4-3) in the liquid crystal composition is preferably 85% and less by weight, more preferably 65% and less by weight, and further more preferably 55% and less by weight in order to minimize optical anisotropy of the liquid crystal composition and to further lower the lower limit of temperature exhibiting a nematic phase, respectively based on the total weight of the liquid crystal composition.
Next, we explain in detail physical properties of the compositions which is constituting the present invention.
A display using liquid crystal composition having the upper-limit of temperature exhibiting a nematic phase lower than 70xc2x0 C. and the lower-limit of temperature exhibiting a nematic phase higher than xe2x88x9220xc2x0 C. has limitation as to the surrounding temperature for use. The display may lose its function, because it becomes unable to display especially in case of outdoor use. For this reason, the nematic phase range of the liquid crystal composition is preferably 70xc2x0 C. or more for the upper-limit of temperature of 70xc2x0 C. and xe2x88x9220xc2x0 C. and less for the lower-limit of temperature.
In case of using a liquid crystal composition having optical anisotropy larger than 0.07 measured at 25xc2x0 C. and using wave length of 589 nm, display in white may become a little yellow in the reflection-type AM-LCD. For this reason, optical anisotropy would be preferably 0.07 and less.
All compounds used in the liquid crystal composition of the present invention are known. For example, the compounds of formula (1-2) are described on their synthetic method in Japanese Patent Kokai Syo 58-170733 (1983). Japanese Patent Kokai Hci 2-233626 (1990) describes the synthetic method for the compounds expressed by formula (2-1) as an example of the compounds expressed by formula (2), for the compounds expressed by formula (3-1-1) as an example of the compounds expressed by formula (3-1), for the compounds expressed by formula (3-2-1) as an example of compounds expressed by formula (3-2), for the compounds expressed by formula (4-1-5) as an example of compounds expressed by formula (4-1), and for the compounds expressed by formula (4-2-2) as an example of the compounds expressed by formula (4-2), respectively. Japanese Patent Kokai Syo 56-135445 (1981) describes the synthetic method for the compound expressed by formula (4-3-1) as an example of the compounds expressed by formula (4-3). As illustrated above, each compound constituting the composition of the present invention could be synthesized by referring prior reference.
The liquid crystal composition of the present invention may comprise liquid crystal compounds except those expressed by formulae mentioned above in such a suitable range that the object of the present invention is not spoiled. The liquid crystal composition of the present invention is prepared by a conventional method. The method is generally mixing various compounds and dissolving each other at high temperature. A chiral dopant such as cholesteric nonanoate may be added to the liquid crystal composition of the present invention, for purposes of adjusting a necessary twist angle which is formed by inducing a spiral structure to a liquid crystal molecule. The liquid crystal composition of the present invention can also be used for guest-host mode by adding dichroic dye such as melocyanines, styrils, azos, azomethines, azoxys, quinophthalones, anthraquinones, and tetrazines; or can also be used for a polymer-dispersion type liquid crystal display device or for a birefringence controlling mode, and dynamic scattering mode; or can also be used for in-plane switching system.